1. Field of the Invention
This invention relates to the technique of suppressing noise from a number of noisy low-level signals to attain a higher-level signal to noise ratio prior to summing.
2. Description of the Prior Art
Seismic exploration operations are frequently conducted using surface-coupled acoustic sources whose energy output is relatively weak. Two typical methods involve either weight dropping or injection of a swept-frequency chirp signal or "sweep" into the ground. Other methods involving weak impulsive sources are known such as pneumatic sources and exploding-gas guns. Seismic energy from the sources are received by detectors and are recorded by suitable equipment as time scale recordings.
Using swept-frequency sources as an example, it is customary to inject into the ground a number of sweeps such as 16. Received signals from the various sweeps are then summed. the received swept-frequency signals are plagued with non-source generated spectrally-overlapping noises. The unwanted noises have amplitude spectra that fall within the spectrum of the swept-frequency signal itself. The unique phase spectrum of the swept-frequency signal provides some degree of phase discrimination against the noise when several sweeps are summed. Additionally, the noise tends to have a random occurrence from sweep to sweep so that during summing some benefit is derived from random noise cancellation. But the amplitude of the noise signals may be many orders of magnitude greater than that of the desired signal so that adequate noise rejection cannot be achieved by phase discrimination alone after summing several sweeps.
The undesired noise may consist of random, short-duration spikes a few milliseconds long, such as might be generated by footfalls of man or beast. Or the noise may take the form of wave trains of several seconds produced by vehicular or aerial traffic near the detectors. It is necessary that a noise-suppression method be able to recognize and reject both short-duration and long-duration noise without excessive rejection of desired seismic signals.
In the past, a number of methods were practiced to suppress or reject high-amplitude noise prior to summing. Before the advent of digital recording, analog data were recorded on magnetic tape. Increasingly large noise signals gradually saturated the magnetic tape thereby imparting a soft clipping action to over-scaled signals.
Early binary-gain ranging digital recording systems were designed to maintain the signal level within certain limits such as between one eighth and one quarter of full scale. Such systems had a fast clamp rate such as 6000 dB/second but a slow release rate such as 200 dB/second. In effect, a digital AGC was provided. Further AGC action was achieved by summing only the mantissas of the data samples; the gain code was suppressed. As with analog recording, high-level noise signals were maintained within tolerable limits. One such binary gain system is described in U.S. Pat. No. 3,525,948 to Sherer et al. The problem of summing data from several records that have been subjected to AGC action is that the seismic signals are not necessarily displayed in their true relative amplitude relationship.
Another noise-blocking method is disclosed in U.S. Pat. No. 3,744,019 to Schmitt, assigned to the assignee of this invention. Schmitt provides a manually-set threshold selector so that signals that exceed a specified level are attenuated or clipped. Since the amplitude threshold is operator-selectable, much depends upon the judgement of the operator. Once set, the threshold selector is unable to cope with changing noise conditions as a function of time during the recording period of 15 to 20 seconds for any one sweep nor is it always practical from an operating standpoint to change the threshold setting between sweeps.
A method in common use with modern instantaneous floating point (IFP) recording systems is the so-called diversity summing or, alternatively, diversity averaging technique. Diversity averaging does not attempt to estimate the signal level. It assumes that the signal level is the same for corresponding times on all sweeps for each channel and that noises have random occurrences (that is, diversity) on the ensemble. Diversity averaging is sensitive to short-term impulsive transients and requires some sort of de-spiker to suppress such transients. A method of diversity stacking is disclosed in U.S. Pat. No. 3,398,396 to Embree.
Another recent noise-blocking method is disclosed in U.S. Pat. No. 3,924,260 to Braham and Kiowski. In this system, model or reference gain factors are determined individually for corresponding time increments on a per-channel basis. The gain factors associated with incoming normalized seismic data samples are compared to the reference gain factor. When a predetermined difference between the incoming and reference gain factors is exceeded, the incoming signal is assumed to be undesired noise and the signal is rejected. The noise rejection feature extends for an adjustable desired time interval to insure that the entire envelope of the objectional noise signal is rejected. If the observed gain factor of the incoming signal falls within a 2:1 ratio of the reference gain factor, the reference gain factor may be updated to equal the observed gain factor.
The system of Braham et al. has an advantage over earlier systems in that the reference gain factor may be updated to fit the changing conditions that are necessarily encountered during the course of a survey. There are certain disadvantages however. The reference gain factor is generated by examining the normalized gain for each sample within an incremental time window. Whenever the normalized gain of a subsequent sample is less than the gain of a previous sample, the lower gain of the subsequent sample becomes the reference gain. Clearly, in the presence of high-amplitude noise, during an initialization period, the reference gain factor is driven to the minimum gain that is associated with the highest-amplitude sample which may in fact be noise. The system is, therefore unduly sensitive to extremes of noise signal levels.